1. Field of the Invention
The present invention relates to composite plating apparatuses which form a composite plating film on the inner surface of a hollow section in a workpiece such as a cylinder block of an engine.
2. Description of the Related Art
Among various examples of the conventionally-known cylinder blocks of internal combustion engine are one where the inner surface of each cylinder, functioning as a sliding surface for a piston, is die-cast integrally with the cylinder block and provided with a composite plating film of Ni (nickel) and SiC (silicon carbide). The Ni/SiC composite plating film is formed by co-depositing SiC in a metal-phase Ni matrix and acts to enhance an abrasion-resistant characteristic of the cylinder inner surface.
Japanese Patent Laid-open Publication No. HEI-7-118891 discloses a surface processing apparatus performing a high-speed plating process, in accordance with which a composite plating film can be formed on the inner surface of a cylinder at an increased speed by compulsorily passing composite plating liquid along the cylinder inner surface. FIG. 18 hereof shows a Ni/SiC composite plating process performed by the disclosed surface processing apparatus.
According to the process of FIG. 18, a cylindrical electrode 102 is provided in a cylinder opening 101 of a cylinder block 100 with a surrounding gap S so that the gap S forms an annular passage 104 between the outer surface of the electrode 102 and the inner surface of the cylinder. The composite plating liquid is caused to first flow in an upward direction as denoted by arrow (1) and then turn inwardly around the top of the electrode 102 to flow in a downward direction as denoted by arrow (2). By energizing the electrode 102 and cylinder block 100 while continuing the flow of the composite plating liquid in the directions of arrow (1) and arrow of (2), a multiplicity of SiC particles 106 are co-deposited in a Ni matrix 105 to provide composite plating film 107.
However, the composite plating film 107 has a drawback as shown in FIG. 19. Namely, because the composite plating liquid flows in the arrowed upward direction along the annular passage 104, a great number of the SiC particles 106 are undesirably co-deposited in an upstream (lower in the figure) portion of the Ni matrix 105. Thus, as the composite plating liquid flows upward (downstream), the number of the SiC particles 106 in the liquid would significantly decrease and the amount of the co-deposited SiC particles 106 in the liquid would gradually decrease. This would present the inconvenience that the resultant composite plating film presents a low abrasion-resistant characteristic in the downstream portion of the cylinder.